Method of forming a soldering jig from polyimide plastic

ABSTRACT

A method of soldering using a polyimide plastic jig where two parts to be joined are located in the jig and are soldered while in the jig.

United States Patent [1 1 Schimmer et a1.

Apr. 2, 1974 METHOD OF FORMING A SOLDERING JIG FROM POLYIMIDE PLASTIC Inventor's: Rigobert Schimmer; Horst Gesing,

both of Belecke; Wolfgang Beerwerth, Neheim-l-lusten; ,Iurgen Messerschmidt, Belecke, all of Germany Assignee: Licentia Patent-Verwaltungs GmbH,

Frankfurt, Germany Filed: June 29, 1971 Appl. N0.: 157,854

Related US. Application Data Continuation-impart of Ser. No. 62,994, Aug. 11, 1970, abandoned.

Foreign Application Priority Data Aug. 11, 1969 Germany 1940769 US. Cl 29/493, 228/56, 269/286,

' 269/321 WE Int. Cl B23k 5/22, 023k 31/02 Field of Search 269/285, 286, 321 WE;

OTHER PUBLICATIONS Vespel Precision Parts from DuPont Polyimide Resins, 1967, 12 pages, Scientific Library Primary Examiner-.l. Spencer Overholser Assistant ExaminerRona1d .1. Shore Attorney, Agent, or FirmSpencer & Kaye [57] ABSTRACT A method of soldering using a polyimide plastic jig where two parts to be joined are located in the jig and are soldered while in the jig.

2 Claims, 3 Drawing Figures 1 METHOD OF FORMING A SOLDERING JIG FROM POLYIMIDE PLASTIC CROSS-REFERENCE TO RELATED APPLICATION The present application is a continuation-in-part application of copending US Pat. application Ser. No. 62,994, filed Aug. 11th, 1970, by the same inventors for a Jig. Application Ser. No. 62,994'was abandoned subsequent to the filing of the present applicatron.

BACKGROUND OF THE INVENTION hold the parts rigidly together.

When high demands on the accuracy of the soldering is necessary, such as is the case in certain electrical applications, for instance semiconductor technology, the soldering jigs must themselves satisfy high requirements for dimensional accuracy and stability. Such soldering jigs can only be made of materials which suffer no ill'effects at the temperature of the molten solder and which do not react with the melt. Neither should there be reactions of the solder with the material of the soldering jig, nor should there be a destruction of the soldering jig by reactions between solder and soldering jig at the requisite soldering temperatures and the usually strongly reducing conditions in the soldering furnace. Additionally, a semiconductor soldering jig must not have a negative influence on the electrical properties of the semi-conductor elements. For instance, it must not cause instability in blocking ability, such as movement and very high reverse currents in the voltage-current curve of a blocking characteristic.

It is known to make soldering jigs of carbon, metals and ceramics. The use of these materials, however, gives rise to certain disadvantages.

Soldering jigs of carbon do not maintain their dimen- 1 sions over extended periods of use, because the carbon tends to chipEspecially in complex soldering jigs, this chipping can lead to loss of accurate holding of the parts to be soldered together and thus to unusable end products. This problem is especially critical in semiconductor technology. i

- Soldering jigs of metal are expensive to manufacture and this problem is especially critical where many jigs are necessary. Each jig must be separately worked by boring, turning, milling or grinding processes. Metal soldering jigs canalso damage the parts to be soldered I together, especially when the parts are of delicate construction. Thus, the applicability of soldering jigs of metal is considerably limited.

Inexpensive soldering jigs of ceramic materials, when unglazed,- have the disadvantage of comparatively rough surface. Moreover, such jigs can be difficult to produce, because the shrinkage during firing can not be accurately predicted. Application of glaze further worsens the problem of obtaining accurate jig dimensions. It is also extremely difficult to adjust the dimensions of a fired ceramic jig.

SUMMARY OF THE INVENTION An object of the present invention, therefore, is to provide a soldering jig which does not have the abovenoted disadvantages of jigs made of carbon, metal or ceramic, which is easily and economically producible,

even in large quantities, which maintains its dimensional accuracy over long periods of usage, which has a smooth surface, and which does not chip during use.

This as well as other objects which will become apparent in the discussion that follows are achieved according to the present invention by a soldering jig made of a plastic resistant to high temperatures and reducing conditions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational section including a jig according to the invention, viewed from the line 11 of FIG. 2.

FIG. 2 is a sectional view from the line 22 of FIG. 1.

FIG. 3 is a view as in FIG. 1 of a jig being used for a soldering operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Since suitable heat-resistant plastics, for example the polyimides, are relatively expensive, it is preferred to combine the jig of the invention with structures of less expensive materials where possible. Thus, when the jig of the present invention is beingused for the soldering of semiconductor components, it is preferred to make only the receptacle for the semiconductor wafer of heat-resistant plastic, for example polyimide. The support for the polyimide jig may be made of a less expensive material, for example nickel'plated steel.

Thus, referring to FIGS. 1 and 2, generally cylindrical, nickel-plated steel support I has arms with protrusions 3 which bear against the jig 2 of high-temperature resistant plastic to grip it and hold it in assembly. Jig 2 is made, for example, of polyimide.

The bearing force of protrusions 3 against jig 2 is a result of the three arms of support 1 being forced to de- Referring now to FIG. 3, the assembly of jig and support of FIGS. 1 and 2 is being used for soldering together a semiconductor wafer 6, an electrical current lead 4, and a cup-shaped housing 7. Discs 5 of solder are shown in place. Jig 2 centers the semiconductor waferfi relative to the internal diameter of housing 7 using the inner diameter of its wafer receptacle. Additionally, lead 4 is centered relative to the wafer. The entire unit, with solder discs' 5, is placed in a soldering 3 4 furnace for yielding an assembly of lead 4, wafer 6, and housing 7. ii

As is apparent from FIG. 3, support 1 functions as a handle for the insertion of jig 2 into housing 7 and for its subsequent removal after soldering. Support 1 addi- 5 tionally functions to maintain lead 4 in a position exg tending vertically upwards from wafer 6; thus the weight of the lead is made to bias the unsoldered sequence, solder-wafer-solder, against the floor of hous- A The polyimides are obtained by condensation reacing 7; additionally, natural flexure of lead 4 is pre- 10 tions ofaromatic tetracarboxylic acids, for example pyvented, thus eliminating the possibility of the tippingromellitic acid, or their anhydrides, for example pyroover of the entire unit in the soldering furnace. mellitic dianhydride, with aromatic diamines, for exam- When making soldering jigs according to the invenple 4,4-diamino-diphenyl ether. Following condensation, for example, by casting or compression methods, tion, polymers of the following general type are obit is generally not necessary to subject the jigs to a subtained:

(i ll 0 O sequent dimensional finishing process, because the The density of polyimides is from 1.3 to 1.7 grams plastic surface has anatural surface smoothness and diper cubic centimeter. They can undergo long-term mensional accuracy. Thus, the soldering jigs according loading at temperatures up to 350 C and, when used to the present invention are advantageously different for shorter periods of time, can be loaded at temperafrom solderingjigs of carbon, metal, and ceramic. A retures up to 500 C. Their ability to withstand reducing action between the molten solder and the plastic which conditions is evident in that they are resistant, for all comprises the soldering jig according to the present inpractical purposes, to treatment at 300 C, one atmovention does not occur. spherepressure, in an atmosphere composed of 20% The plastic materials used for making the soldering hydrogen and 80% nitrogen; their weight loss after 100 jigs of the present invention do not show the disadvansoldering cycles each having a 30 minute length is tages listed above for carbon, metals and ceramics. lt below 1%.

is very easy to make test jigs and small quantities ofjigs Among other classes of the polyimides are, for examby working the plastic with conventional mechanical ple, the polyamide-imides. These are copolymers, methods such as turning. solderingjigs made according which contain imide and amide groups. These copolyto the present invention do not damage delicate surmers are, however, less heat-resistant.

faces. The jigs themselves do not, for all practical pur- The following soft solders have been used successposes, undergo dimensional changes during extended fully to join parts held in polyimide soldering jigs acuse. Mass production of soldering jigs according to the cording to the present invention: lead-tin, lead-indium, present invention can be made by compression or castlead-indium-silver. The preferred solder contains 70 ing methods. Either of these methods makes it possible weight-% lead and 30 weight-% indium. Soldering can to obtain jigs having excellent surface smoothness and be effected with and without flux. A suitable flux is codimensional accuracy at low price. 5 lophony (abietic acid). Soldering temperature for the Especially suited as the plastic material for soldering preferred solder is 320 C. The soldering is carried out jigs according to the present invention are the polyiin a forming gas atmosphere of 20% hydrogen and 80% mides. Among these, a semi-finished part sold under nitrogen.

the name Vespel" has been used with success. Vespel To make the jigs of the present invention by commay be obtained from DuPont de Neinours lnternapression molding, for example pre-condensed polytional 8A.; 81, Route de l'Aire; Geneva, Switzerland. imide molding material in powdered form, for instance Among the Vespels, it is preferred to use Vespel SP-l. M 33, Type A and Type B of the Societe des Usines Chimiques, Rhone Poulenc, possessing a bulk density Another suitable polyimide is, for example, polyof about 0.5 g/cm", is suitable. Type A is used for moldimide molding material M 33, a product of Societe des .ing with unfilled resin or with a filler fraction less than Usines Chimiques, Rhone Poulenc; 22, Avenue Mon- 40 weight-%, while Type B is used when the tiller fractaigne; Paris 8, France. tion is from 40 to 80 weight-percent. Suitable fillers The term polyimide designates a class of chemical are, for example, silica powder, glass fiber dust, mica, compounds in which one nitrogen atom is bonded to and rock dust. Molding pressures are between 50 and two carbonyl groups in a cyclic system according to the 400 kiloponds/cm molding temperatures between following structural formula, where the indicated 230 and 300C, and molding time about 10 minutes. bonds between the nitrogen atom and the carbon atoms Shrinkage of a molding material without fillers is about of the carbonyl groups are termed imide bonds: 1 percent, while with a material containing fillers the shrinkage is less than 1 percent.

After molding, the jigs'are subjected to an after-bake process involving heating for 24 hours at 250C or short heat-shocks of about to minutes at temperatures around 300C. During this after-bake process there is an additional small shrinkage of about 0.1 percent.

Steel is a suitable material for the molds used for molding. The inner surfaces of the mold are polished and preferably chrome-plated.

Polyimide molding material M 33 contains no volatile components.

In casting methods, the polyimide is cast before polycondensation. A casting and mold temperature of 50-300C is used. Curing is carried out at 200-400 C. A shrinkage of about 1 percent occurs.

When soldering semiconductor components designed for high blocking voltages, for example thyristors, using soldering jigs of polyimide, especially jigs made from the intermediate resin Vespel, it has been found that the resulting components exhibit instability in their blocking behavior, if the soldering jig is not given a special preliminary treatment. These negative influences are especially noticeable when a soldering jig has been stored'for a considerable time in air; this is probably due to moisture which is collected by the polyimide. At equilibrium at 50 percent relative humidity, the polyimide Vespel SP 1 has a moisture content amounting to 1.0 to 1.3 percent by weight. Such water contents can actually arise under normal manufacturing conditions; for example, equilibrium may be reached when the jig is stored overnight or when a jig is not under continuous use. Even after a storage of just several hours, significant instabilities in blocking behavior can be noticed, such as movement of the characteristic, and high reverse currents. Since a constant, high quality and high production yield is very important for semiconductor components, these negative influences are extremely disturbing. Consequently, soldering jigs of polyimide, especially of the polyimide Vespel, must be subjected to special treatment before and during production runs. The following treatments have been found to overcome this problem of instability.

When using a flux, such as colophony, and complex soldering jigs, such as are used in the semiconductor technology, a cleaning of the soldering jigs is necessary after a soldering. A combined trichloroethylene and isopropanol cleaning has proven to give excellent results. 7

This cleaning is initiated with a 2 to 3 minuteplacement of the jig in vaporous trichloroethylene, followed by an ultrasonic cleaning in hot trichloroethylene. Then the jig is again placed in the vaporous trichloro ethylene for 2 to 3 minutes. Finally, the jig is treated in cold isopropanol for a further ultrasonic cleaning.

The cleaning is followed by a heating of the jig in a drying chamber under a nitrogen atmosphere with a flow-through rate of 300 standard liters of nitrogen per hour. The temperature and pressure for the standard liter are 0 Centigrade and 760 mm Hg. The heating begins with a 90 minute heating of the jig to 280C, maximum 290C. Subsequently, the jig is cooled to 120C, minimum 100C, in 3 hours. The nitrogen must flow during the entire heating and subsequent cooling. This heating program does not disturb normal production, since the heating and cooling may be done in the course of a night. The program must be co ntrofld such that the next soldering process begins right at the time when the jig temperature has sunk to 120C. Jigs treated in this manner exercise no negative influences on the blocking behavior of soldered semiconductor elements.

Preferably, a jig is kept continually in use, since as above mentioned a storage of only several hours in air leads to significant instabilities in the blocking behavior of semi-conductor elements.

The ultrasonic cleaning apparatus used for the above-described cleaning treatments in trichloroethlandsExernplary parameters for the above-described cleaning and heating process are as follows: temperature and pressure of the vaporous trichloroethylene equal 87,2C and 760 mm Hg, respectively; temperature of the hot trichloroethylene equals 55C; temperature of the cold isopropanol equals 25C; and dimensions of the drying chamber equal 95 cm long, cm

wide, and 60 cm high.

Exemplary parameters for making a jig as illustrated in the drawings and having a maximum outer diameter of 2 centimeters are as follows: Molding material M 33, Type A, mixed with 20 weight-percent silica powder as based on the total weight of the mixture; molding pressure of 200 kiloponds/cm molding temperature of 250C; molding time equals 10 minutes; after-bake process of 24 hours at 250C.

It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. The method of soldering using a polyimide plastic jig comprising the steps of forming a soldering jig from polyimide plastic, locating in the jig two parts to be joined, and soldering the two parts together while lo- *cated in the jig.

2. A method as claimed in claim 1, wherein the steps 9 s ds ns s t s sd fifj fif fim 

1. The method of soldering using a polyimide plastic jig comprising the steps of forming a soldering jig from polyimide plastic, locating in the jig two parts to be joined, and soldering the two parts together while located in the jig.
 2. A method as claimed in claim 1, wherein the steps of soldering is effected in a furnace. 